A switched-mode power supply (SMPS) utilizes energy storage in the magnetic field of an inductance, coupled with switching action from a supply source, to regulate an output voltage, current, or electric power. SMPSs are much preferred over linear power regulation, due to the relatively poor efficiency of linear power conversion systems. Simple SMPSs of early designs often used a diode for flywheel conduction when the switch connecting the magnetizing inductance to the source is switched off. In improving the efficiency of SMPSs, however, conduction diodes have been replaced by switching transistors such as metallic-oxide semiconductor field effect transistors (MOSFETs), or insulated gate field effect transistors (IGFET), which are synchronously switched to provide the necessary blocking and conduction functions at appropriate times. These transistors, which have fairly consistent “on” resistance when switched on, made convenient current sensing resistances to sense the current passing through the magnetizing inductance. However, as the on-resistance of these switching transistors has continued to decrease, the small resistance of circuit board conductors becomes significant as it is on the order of the on resistance of switching transistors. The effect of circuit board conductors becomes especially of concern in multi-phase converters where the distances between the switching transistor through which current is sensed for each phase circuit and the sensing circuit varies, resulting in different resistances for each phase which can lead to imbalanced current among the phases.
Accordingly, there is a need for a current sensing circuit that substantially reduces sensing errors in switching power converters due to circuit board variations.
Those skilled in the field of the present disclosure will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. The details of well-known elements, structure, or processes that would be necessary to practice the embodiments, and that would be well known to those of skill in the art, are not necessarily shown and should be assumed to be present unless otherwise indicated.